Microelectronic devices can be electrically joined to other circuitry so as to send and receive information through input/output ("I/O") connectors including numerous individual input/output lines. Until recently, these connectors typically took the form of rows or arrays of conductive pins, generally referred to as pin grid array ("PGA") technology. For some time, microelectronic device manufacturers have been searching for device joining solutions which would allow them to reduce the spacing between adjacent I/O lines. Device manufacturers have also been concerned with enhancing the electrical device characteristics to aid in increasing the overall operation speed, or frequency at which the devices may reliably work. To meet these concerns, manufacturers have moved toward smaller joining solutions, such as perimeter leaded quad flat packs ("QFPs"). Since then joining solutions with even smaller distances or "pitch" between adjacent I/O lines have proved to be necessary. Many manufacturers are now using solder balls arranged in rows or arrays on the bottom side of the devices to interconnect the microelectronic devices to supporting substrates. Typically, this solution is referred to as ball grid array or bump grid array ("BGA") technology.
BGA devices provide a savings in interconnection area for each device. Moreover, greater numbers of BGA devices may be mounted in a given area of a circuit substrate, allowing smaller substrates to be used which in turn tends to minimize signal path lengths between connected devices. This, in turn, reduces signal propagation delays and other detrimental effects such as stray inductance and capacitance.
The proliferation of BGA-type devices has created a need for test sockets which are used to examine the devices prior to shipment to customers. There is a similar need for sockets which can permanently connect BGA devices to supporting substrates. Each bump lead may be entirely comprised of an electrically conductive, heat-activatable bonding material, such as solder. Alternately, the bump leads may be comprised of a combination of a solid core sphere (typically made of copper or nickel or a combination thereof) with a electrically conductive, heat-activatable bonding material around the exterior of the sphere. Further still the bump leads may be plated bumps, typically copper, nickel or gold. The size of a BGA solder ball is typically between about 0.1 mm and about 0.8 mm (5 and 30 mils) in diameter.
One problem encountered in socketing BGA devices is that the bump leads on a single device typically range in diameter width, height and center position. Each BGA socket connector must be able to make a good electrical connection with its respective bump lead while accommodating for these varying factors. Sockets should be also able to accommodate BGA having a distance between adjacent balls of about 1.5 mm or less. The ability to accommodate even smaller pitches, down to about 0.5 mm or less, will become more significant in the near future. These factors, taken together, pose a formidable challenge. In particular, it is difficult to provide for a range of ball diameters in a small contact.
When the device is in operation, another problem that arises is that the device and its supporting substrate typically expand and contract at different rates and at different times, thereby repeatedly stressing the interconnections between them. This can cause the permanently attached bump leads (such as soldered connections) to become unreliable. A BGA socket should also be able to accommodate for these thermal expansion stresses.
A variety of solutions have been put forth to deal with the aforementioned problems. U.S. Pat. Nos. 5,196,726 and 5,214,308 both issued to Nishiguchi et al. disclose a BGA type approach in which bump leads on the face of the chip are received in cup-like sockets on the substrate and bonded therein by a low-melting point material. U.S. Pat. No. 4,975,079 issued to Beaman et al. discloses a test socket for chips in which dome-shaped contacts on the test substrate are disposed within conical guides. The chip is forced against the substrate so that the solder balls enter the conical guides and engage the dome-shaped pins on the substrate. Enough force is applied so that the dome-shaped pins actually deform the solder balls of the chip. Hill et al., Mechanical Interconnection System For Solder Bump Dice, 1994 ITAP And Flip Chip Proceedings PP. 82-86, discloses a test socket for flip chip devices with solder bumps. The socket has rough, dendritic structures on contact pads; here again, the chip with the solder bumps thereon is forced into the engagement with the rough, dendritic structures so as to make temporary contact for testing.
A further example of a BGA socket may be found in U.S. patent application No. 08/254,991 filed Jun. 7, 1994, commonly assigned to the assignee of the present invention, which deals effectively, but specifically differently, with the problems associated with socketing a BGA device. The '991 application discloses a sheet-like connector having a plurality of holes. Each hole is provided with at least one resilient laminar contact extending inwardly over a hole. The bump leads of a BGA device may then be advanced into the holes so that the bump leads are engaged with the contacts. The assembly can be tested, and if found acceptable, the bump leads can be permanently bonded to the contacts.
Other examples of connector solutions include U.S. Pat. No. 5,380,210 issued to Grabbe et al. discloses an area array connector having leaf spring-like deformable contacts which are disposed within individual module bodies. The module bodies are then placed within an array of openings in a module holder so that the spring contacts may be compressibly placed between and wipe against the opposing conductive pads of a device and a supporting substrate. U.S. Pat. No. 5,360,347 issued to Irlbeck discloses a compressible connector array assembly employing a plurality of individual connectors bonded together and arranged within a frame. U.S. Pat. Nos. 5,152,695, 5,173,055, and 5,228,861 all issued to Grabbe disclose alternate area array connection systems using deformable contacts. U.S. Pat. No. 5,006,792 issued to Malhi et al discloses a test socket in which a substrate has an exterior ring-like structure and numerous cantilever beams protruding inwardly from the ring-like structure. Contacts are disposed on these cantilever beams so that the same can be resiliently engaged with contacts of a chip when the chip is placed in the socket.
Despite all of these efforts in the art, still further improvements would be desirable.